1. Field of the Invention
This invention relates to a polymer-dispersed liquid crystal composite (hereinafter referred to as a PDLC composite) and, more particularly, to a PDLC composite based on an electron-rich alkenethiol polymer which is radiation curable and can be cast into a thin film.
PDLC composite systems are typically thin films composed of droplets of liquid crystals dispersed in a polymeric matrix. PDLC composite systems are of interest because they can be switched between an opaque state (light scattering) and a transparent state (light transmitting) by the application of a suitable electric or magnetic field, or by a significant change in temperature. A composite with these properties is possible because the liquid crystal is birefringent. It possesses two refractive index values: one value when the molecules comprising each droplet are randomly oriented, and a different value when the molecular axes of the molecules are aligned with respect to each other. Alignment of the axes occurs when a voltage, magnetic field, or significant change in temperature is applied to the PDLC composite. The liquid crystal and polymeric matrix are chosen so that the refractive indices of these two components match when the magnetic field, electric field or temperature is significantly varied, thus resulting in a transparent material. When the refractive indices of the polymer matrix and the LC are not identical or nearly identical, the dispersed LC droplets act to scatter light and an opaque material results.
PDLC composites are being used for, among other things, light valves, filters, shutters, electro-optic switches, information display arrangements (like the LCD watch), switchable windows and projection television.
2. Description of the Related Art
Various polymer matrices for PDLC systems have been proposed. For example, U.S. Pat. No. 4,728,547 issued to Vaz et al. discloses an optically responsive film comprising liquid crystals dispersed in a UV-curable ene-thiol, particularly thiol-allyl, polymer matrix. Suitable polymers are based on the reaction product of triallylisocyanurate and/or other suitable di- and tri-allyl ethers or esters and one or more polythiols and a photoinitiator such as benzophenone.
U.S. Pat. No. 4,869,847 issued to Leslie et al. discloses a polymer matrix for liquid crystal technology based on monomers derived from acrylic acid and methacrylic acid such as acrylates and methacrylates. Similarly, U.S. Pat. No. 5,011,624 issued to Yamagishi et al. discloses the use of three classes of acrylate monomers: esters derived from acrylic acid, esters derived from methacrylic acid, and amides derived from acrylic acid for use in PDLCs.
However, these prior art films often exhibit poor mechanical properties. The films have low tensile strength and thus are subject to breakage. One reason for this is that the liquid crystal material is an aggressive solvent and thus weakens the polymer matrix.
In addition, polymer matrices based on acrylates often gel too quickly, causing the formation of droplets that have an undesirable morphology. Ene-thiol matrices, on the other hand, often contain ionic containments which adversely affect the film's electrical, theological and physical properties. Ene-thiol matrices also cure slowly, which increases the processing time of the films.
Thus, there is a need for a polymer matrix which has good mechanical properties, especially high tensile strength, faster processing time, low ionic contamination, and provides film with good optical contrast between the "on" and "off" states.